[eru]

Unions in Germany are sort-of mandatory. In the sense that whatever the unions negotiate also has legal consequences for non-union members. See https://de.wikipedia.org/wiki/Tarifbindung

About legality: have a look at https://en.wikipedia.org/wiki/No-show_job and https://www.nytimes.com/2012/03/22/nyregion/no-show-jobs-tai...